Process for treating the surface of a part made of aluminium or aluminium alloy or of magnesium or magnesium alloy

ABSTRACT

The invention relates to a method for the surface treatment of a part made from aluminum or aluminum alloy or from magnesium or magnesium alloy, comprising a step of treatment by oxidation of said part and a step of applying an aqueous composition to the surface of said part.

The present invention falls within the field of the surface treatment of parts made from aluminum or magnesium alloy with a view to protecting them against corrosion.

More particularly, the present invention relates to a method for the surface treatment of a part made from aluminum or aluminum alloy or from magnesium or magnesium alloy using an aqueous composition for the surface treatment. The invention also relates to a part made from aluminum alloy or from magnesium alloy obtained by such a method.

Aluminum or magnesium alloys are widely used in the aeronautical field for their low density and their good mechanical properties. However, they must be protected by surface treatments so as to prevent them from corroding.

One method which is widely used at the present time for the protection of parts made from aluminum alloys, and more generally from light alloys, is anodization, also called anodic oxidation. The parts to be treated are immersed, with a suitable counter-electrode, in a bath containing an electrolyte, in which an anode current is applied. A protective layer, called an anode layer, is then formed on the surface of the part, which layer is composed mainly of oxides and hydroxides resulting from the elements of the substrate, and in some cases elements from the electrolyte. However, the anode layers alone are not sufficient to effectively protect the parts against corrosion, because they are generally porous, and are therefore susceptible to corrosion. They therefore need to be painted or sealed. The sealing treatments proposed for this purpose by the prior art, and which have been implemented for many years, use potassium or sodium dichromate.

Furthermore, in the event that electrical conductivity is required for the part, the combination of anodization and sealing is replaced by a chemical conversion treatment. Currently, this treatment is most generally carried out by means of a solution based on chromium trioxide, for example by the solution marketed under the name “Alodine 1200” by the company Henkel. The anti-corrosion protection imparted by the conductive layer thus formed is less than that obtained by anodization and then sealing, but it can also serve as a bonding base for a paint.

These methods, whether post-anodization sealing or chemical conversion, conventionally use substances based on hexavalent chromium, which are classified as CMR (carcinogenic, mutagenic or toxic for reproduction) and which are now subject to authorization by the European REACH regulation.

In order to replace these methods, various solutions have been proposed by the prior art which do not use hexavalent chromium.

Regarding the methods for sealing the anode layers formed by surface anodization of the parts, which aim to close the pores and thus make the anode layer chemically inert, the method most commonly used at this time is hydrothermal sealing (HTS), or hot water sealing (HWS). The anodized part is immersed in boiling water or water which is near its boiling point, usually at 98° C. The pore surface is then partially converted to aluminum hydroxides, in particular boehmite (AlOOH) and pseudoboehmite (AlO(OH)), which block the pores and thus improve the corrosion resistance of the anode layer. The main drawbacks of this solution are high energy consumption, as well as high thermal stresses which cause cracks to form in the anode layer. Such a method thus does not make it possible to obtain an anti-corrosion protection which is equivalent to that obtained by solutions based on hexavalent chromium.

Much work has focused on developing sealing solutions based on non-toxic chemical compounds. The methods most often studied use trivalent chromium, that is to say, in the +3 oxidation state. To date, there are thus currently methods on the market for the sealing or chemical conversion of parts made from aluminum alloy based on trivalent chromium, such as the solution marketed under the name SurTec® 650 by the company SurTec®, the solution marketed under the name Lanthane 613.3 by the company Coventya, the solution marketed under the name Metalast by the company Chemetall, or the solution marketed under the name Socosurf TCS/PACS by the company Socomore.

Mention may also be made, for example, of the surface treatment method proposed by document U.S. Pat. No. 6,669,786, which uses several metal ionic species, including at least one in the trivalent state and at least one in the hexavalent state.

However, these methods do not make it possible to meet all of the requirements of the aeronautical field in terms of corrosion protection. In addition, the compositions based on trivalent chromium are capable of generating traces of hexavalent chromium in the layer formed on the part, in particular when they experience aging under humid conditions, such that they themselves may also present an environmental risk.

The present invention aims to remedy the drawbacks of the solutions proposed by the prior art for the surface treatment of parts made from aluminum or magnesium alloy with a view to improving their corrosion resistance, in particular the drawbacks set out above, by proposing a method which uses a composition free of chromium and which makes it possible to replace solutions using chromium with equivalent performance in terms of the corrosion resistance imparted to the treated part.

Additional objectives of the invention are for the method to be easy and quick to perform and low in energy consumption, and for the composition used by the method to be free of any substance which is toxic to living organisms and the environment in general.

The invention also aims to ensure that the corrosion resistance capacity of the treated part is durable over time, and that this treated part has properties of self-healing of the defects formed in the coating present on its surface.

It has now been discovered by the present inventors that, quite surprisingly, such objectives are achieved by a particular combined choice of a small number of constituents in ionic form, which, when applied to the surface of the part to be treated, form an ionic conversion coating on the surface of said part which exhibits a particularly high corrosion resistance, this application being done in a single step, very quickly, in a time less than or equal to 15 minutes, and what is more at ambient temperature.

Thus, according to a first aspect, there is proposed according to the present invention a method for the surface treatment of a part made from aluminum or aluminum alloy or from magnesium or magnesium alloy, with a view to improving its corrosion resistance. This method comprises a step of treating the part by oxidation, then a step of applying an aqueous liquid composition to the surface of the part which is very particularly suitable for use for the surface treatment of a metal part, in particular made from aluminum or aluminum alloy or from magnesium or magnesium alloy. This aqueous composition contains:

-   -   fluorozirconate ions, preferably hexafluorozirconate ZrF₆ ²⁻         ions,     -   molybdate MoO₄ ²⁻ ions,     -   and at least one component selected from lithium Li⁺ ions and         permanganate MnO₄ ⁻ ions.

This composition is also free of chromium, in particular of hexavalent chromium and of trivalent chromium. This means that the composition is substantially free of chromium, that is to say that it does not contain chromium, or only in trace amounts.

Preferably, the composition does not contain hexavalent chromium, not even in trace amounts.

It is accepted in the context of the invention that the composition may contain small amounts of trivalent chromium, for example less than 0.01% by weight.

According to the invention, prior to the step of applying the composition to the surface of the part, the surface treatment method comprises a step of treating the part by oxidation, in particular by micro-arc oxidation or else by anodic oxidation, also called anodization.

The part treated by the method according to the invention also has a dense surface layer, and, when the treatment step is a step of treatment by anodization, the pores of the anode layer which have been formed on its surface during the anodization are advantageously blocked.

The step of applying the composition according to the invention to the surface of the part then seals the anode layer which was formed during the anodization.

The anodization can be carried out according to any conventional method in itself. However, anodization methods which use chromium, in particular hexavalent chromium, are preferably avoided in the context of the present invention, for obvious environmental safety reasons.

The anodization method is chosen in particular from the methods known to those skilled in the art of sulfo-tartaric, sulfo-boric, etc. anodization. In the context of the invention, sulfuric anodic oxidation (SAO) methods are particularly preferred, in particular methods making it possible to form an anode layer on the part having a fine thickness, typically between 3 and 7 μm, commonly referred to as fine SAO. An example of such a method is in particular described in patent document FR 2 986 807. The invention is not, however, limited to such a method and can also be applied, with the same success, to the sealing of anode layers with a greater thickness, for example up to 25 μm. It also applies, with the same success, to the sealing of the layers formed at the surface of the substrates by the micro-arc oxidation technique.

At the end of the anodization step, the treatment method according to the invention may comprise, prior to the application of the composition according to the invention to the surface of the part, a step of rinsing the surface of the part, for example with water, and where appropriate a drying step, such steps being entirely optional.

The step of applying said composition can be carried out according to any method conventional in itself for those skilled in the art. It is preferably carried out by immersing the part in a bath of the composition according to the invention.

Otherwise, it can be done by spraying, pad application, etc.

For this step, the temperature of the composition is preferably advantageously between 10 and 60° C., preferably between 15 and 30° C., and more preferably between 18 and 25° C., in particular approximately 20° C., and more generally at ambient temperature. Such a characteristic proves to be particularly advantageous with regard to the energy expenditure necessary for the implementation of the method according to the invention, which is particularly low.

In preferred embodiments of the invention, the application of the composition according to the invention to the surface of the part is carried out for a time greater than or equal to 5 minutes, preferably between 5 and 60 minutes, and more preferably between 5 and 20 minutes, in particular between 8 and 15 minutes. Here again, such a characteristic proves to be particularly advantageous from an economic point of view.

The method according to the invention makes it possible to form a surface layer on the part which exhibits particularly good, durable corrosion resistance, and a good self-healing capacity.

The part treated by the method according to the invention also has a dense surface layer, and, when the treatment step is a step of treatment by anodization, the pores of the anode layer which have been formed on its surface during the anodization are advantageously blocked.

In particular embodiments of the invention, the method comprises a preliminary step of pretreatment of the part by chemical degreasing and/or chemical pickling, also called a surface preparation step.

Such a preliminary surface preparation step advantageously makes it possible to clean the surface of the part of its dirt, oxides, etc.

Degreasing, like pickling, can be carried out in any manner known to those skilled in the art.

The degreasing can in particular be of the solvent or alkaline type. This is for example an alkaline degreasing, by soaking the part, for example for 20 minutes, in an aqueous bath at a temperature of 60° C. for example, containing the products marketed under the names Turco® 4215 NCLT and Turco® 4215 additive, pH 9, for example at concentrations of 50 g/l and 10 g/l, respectively.

The pickling can in turn be both of the acidic type and of the alkaline type. This is for example an acid pickling, by soaking the part, for example for 5 minutes, in an aqueous bath at a temperature of 20° C. for example, containing the product marketed under the name Turco® Smut Go NC, for example at a concentration of 19% by volume.

Rinses, in particular with demineralized water, and at ambient temperature, can be carried out between the various degreasing and pickling phases, and at the end of this preliminary surface treatment step.

The step of the surface treatment method for applying the composition according to the invention to the surface of the part can be carried out directly after this preliminary pretreatment step. The chemical conversion of the aluminum alloy or the magnesium alloy is then carried out, so as to form a corrosion-resistant surface layer on the part.

In particular embodiments of the invention, the surface treatment method comprises a final step of rinsing the surface of the part, for example with water, and where appropriate a step of drying this surface.

Preferably, the method according to the invention does not include any other sealing step, in particular sealing with hot water, after the step of applying the composition according to the invention to the surface of the part.

The method according to the invention comprises a step of applying, to the surface of the part, an aqueous composition which may thus contain:

-   -   fluorozirconate ions, molybdate ions and lithium ions,     -   or fluorozirconate ions, molybdate ions and permanganate ions,     -   or even, preferably, fluorozirconate ions, molybdate ions,         lithium ions and permanganate ions.

The composition applied to the part by a method according to the invention can advantageously be used, with high performance, both for a post-anodization or micro-arc post-oxidation sealing treatment. High corrosion protection performance is also obtained both on parts obtained by rolling and on parts obtained by machining.

This composition, after application to the surface of a part to be treated, gives said part a corrosion resistance capacity as good as, or even better than, the hot water sealing solutions or the commercial solutions for sealing or chemical conversion based on trivalent chromium which are proposed by the prior art.

It also gives surface self-healing properties to the part. By way of example, no pitting corrosion appears on scratched test pieces treated with the composition according to the invention, after more than 800 hours of exposure to salt spray.

In particular, the part treated by the method according to the invention exhibits only very little pitting corrosion after more than 1000 hours of exposure to salt spray; it also exhibits a very low corrosion current, determined by the method of potentiodynamic polarization curves.

The mechanisms underlying the effect of such beneficial results will not be prejudged here. The treatment of the part by the method according to the invention results in the formation, on the part, of a surface layer which, in addition to aluminum or magnesium, contains zirconium, fluorine, molybdenum, as well as lithium and/or manganese. Within this layer, each of these elements plays a role for anti-corrosion protection, individually but also especially in synergy with the other elements that are present.

Possible mechanisms of action of each individual constituent of the composition according to the invention have been described in the literature.

For example, the fluorozirconate ions, in which a zirconium atom is in complexed form with fluorine atoms, would participate in the formation of a hydrated layer of Al—Zr—O—F on the surface of the part which would increase the hydrophilic nature of the surface of the part and would activate this hydrophilic activity. Molybdate ions, which have a reducible hexavalent anion which forms an insoluble oxide, would have a corrosion inhibiting action within this surface layer. The lithium ions would participate in the formation of a lithium aluminate salt (LiH(AlO₂)₂.5H₂O), which would seal the pores in the surface layer of the part. The permanganate ions would in turn have a self-healing effect by Mn⁷⁺ leaching, transport by the composition and reduction in insoluble form of manganese at the defects present in the surface layer, to form sealing intermetallic precipitates therein.

However, these individual hypotheses do not make it possible to explain the significant performance, in terms of corrosion resistance and self-healing, of the surface layer formed on the parts by treatment using the composition used by the method according to the invention. There was nothing to suggest that, combined, these constituents would make it possible to obtain such high performance in such a short treatment time and, moreover, at ambient temperature, that is to say at a temperature between approximately 18 and 25° C., without the need for heating or without the need to provide any external energy.

Preferably, the composition applied to said part by the method according to the invention has one or more, preferably all, of the following features:

-   -   a hexafluorozirconate ion concentration of between 3.5 and 22         g/l, preferably between 3.5 and 9 g/l;     -   a molybdate ion concentration of between 1.5 and 7 g/l,         preferably between 3.5 and 7 g/l;     -   a lithium ion concentration of between 0.2 and 1 g/l, preferably         between 0.4 and 1 g/l;     -   and/or a permanganate ion concentration of between 3 and 15 g/l,         preferably between 6 and 15 g/l.

The composition applied to said part by the method according to the invention may also contain cerium ions, preferably in the +3 oxidation state. Such a feature further increases the anti-corrosion performance of the treated part, in particular by the formation of precipitates of cerium oxides and hydroxides on the cathode sites of the metal surface, due to a strong local increase in the pH.

The composition applied to said part by the method according to the invention can in particular have a concentration of cerium ions of between 0.01 and 0.2 g/l, preferably between 0.05 and 0.2 g/l.

The composition applied to said part by the method according to the invention also preferably contains nitrate ions. These nitrate ions, which advantageously play an oxidizing role in said composition, thus further improving the anti-corrosion performance of the treated part, can for example be provided in the composition in the form of a salt, for example a lithium salt and/or a cerium salt.

In general, all of the ions contained in the composition applied to said part by the method according to the invention can be provided therein in the form of one or more water-soluble salts.

The cations, in particular of lithium and/or cerium, can for example be provided in the form of sulfate, persulfate, chloride, nitrate, fluoride, acetate, carbonate, etc., or any one of their mixtures. Preferably, they are provided in the form of nitrate, in particular lithium nitrate and/or cerium nitrate.

Thus, the composition applied to said part by the method according to the invention preferably contains lithium nitrate LiNO₃ and/or cerium nitrate, in particular cerium in the +3 oxidation state, Ce(NO₃)₃.

The anions, in particular of fluorozirconate, molybdate and/or permanganate, can for example be provided in the composition in the form of one or more salts of one or more alkali metals, for example in the form of a potassium or sodium salt, or a mixture thereof.

Thus, the composition applied to said part by the method according to the invention may for example contain one or more of the following salts:

-   -   potassium hexafluorozirconate K₂ZrF₆     -   sodium molybdate Na₂MoO₄     -   potassium permanganate KMnO₄.

The composition applied to said part by the method according to the invention may for example have at least one, preferably several, and more preferably all, of the following features:

-   -   a concentration of between 5 and 30 g/l, preferably between 5         and 12 g/l of fluorozirconate salt(s), for example of potassium         fluorozirconate K₂ZrF₆;     -   a concentration of between 2.5 and 10 g/l, preferably between 5         and 10 g/l of molybdate salt(s), for example sodium molybdate         Na₂MoO₄, 2H₂O;     -   a concentration of between 2 and 8 g/l, preferably between 4 and         8 g/l of lithium salt(s), for example lithium nitrate LiNO₃;         and/or a concentration of between 4 and 19 g/l, preferably         between 9 and 19 g/l of permanganate salt(s), for example of         potassium permanganate KMnO₄;     -   and/or, where appropriate, a concentration of between 0.1 and         0.5 g/l, preferably between 0.2 and 0.5 g/l of cerium salt(s),         for example cerium nitrate Ce(NO₃)₃, 6H₂O.

These concentrations are the total concentrations of the salt(s) in question.

In particular embodiments of the invention, the composition applied to said part by the method according to the invention contains one of the following combinations of constituents:

-   -   fluorozirconate ions, preferably hexafluorozirconate ions, in         particular in the form of potassium salt; molybdate ions, in         particular in the form of sodium salt; and lithium ions, in         particular in the form of lithium nitrate. It can for example         essentially contain these constituents, in solution in water;     -   fluorozirconate ions, preferably hexafluorozirconate ions, in         particular in the form of potassium salt; molybdate ions, in         particular in the form of sodium salt; lithium ions, in         particular in the form of lithium nitrate; and cerium ions, in         particular in the form of cerium nitrate. It can for example         essentially contain these constituents, in solution in water;     -   fluorozirconate ions, preferably hexafluorozirconate ions, in         particular in the form of potassium salt; molybdate ions, in         particular in the form of sodium salt; and permanganate ions, in         particular in the form of potassium salt. It can for example         essentially contain these constituents, in solution in water;     -   fluorozirconate ions, preferably hexafluorozirconate ions, in         particular in the form of potassium salt; molybdate ions, in         particular in the form of sodium salt; permanganate ions, in         particular in the form of potassium salt; and cerium ions, in         particular in the form of cerium salt. It can for example         essentially contain these constituents, in solution in water.

A composition applied to said part by the method according to the invention preferably contains, in solution in water:

-   -   a fluorozirconate salt, for example potassium         hexafluorozirconate,     -   a molybdate salt, for example sodium molybdate,     -   a lithium salt, for example lithium nitrate,     -   a permanganate salt, for example potassium permanganate,     -   and, where appropriate, a cerium salt, for example cerium         nitrate.

Preferably, the composition applied to said part by the method according to the invention essentially contains the five constituents listed above.

In the present description, the expression “essentially contains” is understood to mean that the composition contains only these components, or that it also contains other components, but only in trace amounts, in non-working amounts, that is to say which have no effect on the aluminum or magnesium alloy making up the treated part or on the surface layer formed on the surface of said part, prior to the application of the composition according to the invention to this surface, by anodization or another form of oxidation, or during this application.

Preferably, the composition applied to said part by the method according to the invention is substantially free of one or more, and preferably all, of the following components:

-   -   phosphate ions,     -   vanadium,     -   and/or hydrogen peroxide.

In addition and preferably, the composition applied to said part by the method according to the invention substantially does not contain, besides the fluorozirconate ions, other ions containing fluorine. In particular, it is substantially free of any substance which is capable of releasing fluoride ions into the composition, for example ammonium fluoride, tetrafluoroboric acid, etc.

In the present description, the term “substantially free” means that the composition is free of the compound, or that it contains it only in trace amounts, in amounts which are ineffective for the intended application.

The pH of the composition applied to said part by the method according to the invention is preferably between 3 and 7, more preferably between 4 and 6.5, and even more preferably between approximately 5 and approximately 6.

According to another aspect of the invention, a method for the surface treatment of a part made from aluminum or aluminum alloy or from magnesium or magnesium alloy comprises a step of applying, to the surface of said part, an aqueous composition essentially comprising one or more fluorozirconate salt(s) and one or more molybdate salt(s).

Another aspect of the invention relates to a part made from aluminum or aluminum alloy or from magnesium or magnesium alloy obtained by a surface treatment method according to the invention.

This part is coated with a surface layer having a particularly high corrosion resistance as well as self-healing properties. This surface layer contains aluminum or magnesium, as well as zirconium, fluorine and molybdenum, and as well as lithium and/or manganese. It may also contain cerium.

This surface layer is dense and contains uniformly ordered spheroidal aggregates.

When the step of applying the composition to the surface of the part is carried out after an anodization step, the pores of the anode layer which has been formed by anodization are also advantageously closed. The step of applying the composition according to the invention to the surface of the part increases the thickness of the anode layer by approximately 0.1 to 2 μm, depending on the exact composition used.

Furthermore, nothing prevents one of the compositions used by a method according to one of the variant embodiments of the invention described above from being applied, during a chemical conversion treatment, directly to parts not previously treated by anodization.

The features and advantages of the invention will emerge more clearly in the light of the embodiments below, which are provided purely by way of illustration and in no way limit the invention, with the support of FIG. 1 to 4, in which:

FIG. 1 shows photographs of respectively rolled (a/) and machined (b/) test pieces, made from aluminum alloy, anodized and sealed in accordance with the invention with an aqueous composition containing fluorozirconate, molybdate, lithium, permanganate and cerium ions (C4), after 750 hours of exposure to salt spray;

FIG. 2 shows analysis micrographs by scanning electron microscopy of aluminum alloy test pieces anodized and sealed in accordance with the invention, a/ with an aqueous composition containing fluorozirconate, molybdate and lithium ions (C1), b/ with an aqueous composition containing fluorozirconate, molybdate, lithium and permanganate ions (C2), c/ with an aqueous composition containing fluorozirconate, molybdate, lithium and cerium ions (C3), d/ with an aqueous composition containing fluorozirconate, molybdate, lithium, permanganate and cerium ions (C4); in this figure, for each test piece, the image obtained in backscattered electron mode is shown on the left and the image in secondary electron mode is shown on the right;

FIG. 3 shows a photograph of a rolled test piece made from aluminum alloy, anodized and sealed in accordance with the invention with an aqueous composition containing fluorozirconate, molybdate, lithium, permanganate and cerium ions, and marked in an X pattern by a Van Laar tip;

and FIG. 4 shows analysis micrographs by scanning electron microscopy of anodized and sealed aluminum alloy test pieces, marked in an X pattern by a Van Laar tip, before exposure to salt spray (a/), and after 816 h of exposure to salt spray, respectively after sealing with an aqueous composition applied by the method according to the invention containing fluorozirconate, molybdate, lithium, permanganate and cerium ions (b/) and after sealing with an aqueous composition containing only fluorozirconate ions (c/ and d/, at different magnifications).

EXAMPLE 1

AA2024 aluminum alloy test pieces (with the following composition: 1.2 to 1.8% Mg, 0.3 to 0.9% Mn, max. 0.5% Fe, 3.8 to 4.9% Cu, max. 0.25% Zn, max. 0.1% Cr, max. 0.15% Ti, Al for the remaining %), rolled or machined, with dimensions 25×100×3 mm (for microstructural characterizations) and 150×80×3 mm (for salt spray tests) were treated using the method according to the present invention as per the following operating conditions.

The test pieces were first subjected to surface preparation steps. For this purpose, they were successively soaked in the baths below:

-   -   aqueous bath containing Turco® 4215 NCLT 50 (50 g/l) and Turco®         4215 additive pH 9 (10 g/l), at 60° C., for 20 min (alkaline         degreasing)     -   demineralized water at ambient temperature for 5 min (rinsing)     -   aqueous bath containing Turco® Smut Go NC (19% v/v), at 20° C.,         for 5 min (pickling)     -   demineralized water at ambient temperature for 5 min (rinsing).

The test pieces were then subjected to a sulfuric anodic oxidation treatment, in a conventional manner in itself, according to the following parameters:

-   -   200 g/l aqueous sulfuric acid electrolytic bath     -   duration 21.33 min     -   bath temperature 19° C.     -   voltage increase at a speed of 3 V/m in, up to a plateau value         of 16 V, and hold at this plateau value for 16 min.

At the end of these steps, an anode layer was obtained on the surface of the test pieces.

The test pieces were then subjected to a sealing treatment in accordance with the present invention. For this, they were immersed in the following aqueous composition:

K₂ZrF₆ 25 g/l Na₂MoO₄, 2H₂O 5 g/l LiNO₃ 4 g/l KMnO₄ 9.5 g/l Ce(NO₃)₃, 6H₂O 0.1 g/l

The pH of this composition is equal to 6.

Certain test pieces were treated with the same composition, but devoid of Ce(NO₃)₃ (“without Ce”).

The treatment temperatures and times are shown in Table 1 below.

At the end of these steps, a layer with the thickness indicated in Table 1 below was obtained on the surface of the test pieces.

After sealing, the test pieces were directly, i.e. without rinsing, exposed to salt spray for 750 or 1176 h, at a temperature between 15 and 25° C., according to the conditions in accordance with the NF EN ISO 9227 standard.

Each set of conditions was performed in triplicate.

By way of comparative example, test pieces were subjected to sealing by means of the method based on trivalent chromium sold under the name SurTec® 650.

The obtained results, expressed as the number of pits observed on the surface of the test piece after exposure to salt spray, are shown in Table 1 below.

TABLE 1 Results of a salt spray exposure test for anodized and sealed aluminum alloy parts Number Number Layer of pits of pits Temperature Duration thickness after after Sample (° C.) (min) (□m) 750 h 1176 h AA2024 50 8 6/6.2/6.1 6/7/7 10/6/10 rolled AA2024 ambient 8 6.3/6.2/5.6 1/1/1 1/2/2 rolled AA2024 ambient 8 5.3/5.5/5.4 1/1/1 1/1/1 rolled AA2024 50 40 / 3/5/3 / machined AA2024 ambient 8 5.2/5.3/5.2 1/1/1 1/1/1 machined AA2024 ambient 8 7.2/7.4/7.4 1/1/1 1/1/1 machined AA2024 50 8 5.6/3.8/3.5 0/0/1 0/1/3 machined - “without Ce” AA2024 ambient 8 4.2/3.0/2.5 1/1/0 / rolled SurTec ® 650 AA2024 ambient 8 4.6/4.9/5.9 3/3/3 / machined - SurTec ® 650

As can be seen, very satisfactory corrosion protection of the test pieces is obtained for all of the conditions tested. Particularly good results are advantageously obtained for the treatment with cerium at ambient temperature: after only 8 min, the corrosion resistance of the test pieces is particularly good, both for the rolled test pieces and for those which have been machined in bulk. It is in particular similar to, and for machined test pieces even better than, that obtained by the SurTec® 650 commercial method based on chromium proposed by the prior art.

EXAMPLE 2

Rolled AA2024 aluminum alloy test pieces with dimensions 25×10×1 mm were subjected to steps of surface preparation and then anodization as described in Example 1 above.

These test pieces were then sealed using compositions applied by a method according to the invention and described in Table 2 (C1 to C4).

By way of comparative examples, similar test pieces were sealed using compositions not in accordance with the invention, also described in Table 2 (Comp1 and Comp2).

Certain test pieces did not undergo any sealing treatment after anodization (Comp).

The sealing conditions were 15 min at ambient temperature (i.e. approximately 21° C.).

At the end of the sealing treatment, the test pieces were rinsed with water and dried at 60° C. for 10 min.

The thickness of the layer which was formed on their surface is shown in Table 2 below.

TABLE 2 Sealing compositions used K₂ZrF₆ Na₂MoO₄, LiNO₃ KMnO₄ Ce(NO₃)₃, Thickness Composition (g/l) 2H₂O (g/l) (g/l) (g/l) 6H₂O (g/l) pH (□m) C1 12 10 8 — — 6.04 6.9 C2 12 10 8 19 — 5.34 5.9 C3 12 10 8 — 0.2 5.91 6.8 C4 12 10 8 19 0.2 5.94 6.2 Comp1 12 — — 19 — 3.96 5.8 Comp2 — 10 — 19 — 8.44 5.2

The test pieces were subjected to the following tests.

Salt Spray Resistance Test

This test was carried out as described in Example 1 above, for 750 h, for the test pieces treated with composition C4 according to the invention. After 750 hours of exposure to the salt spray, very little pitting corrosion is observed on the surface of the test pieces, as can be observed in the photographs shown in FIG. 1.

For these test pieces, the appearance of 3 corrosion pits per dm² is observed on average after 750 h of exposure to salt spray.

Similar results are obtained for test pieces made from the same alloy and with the same dimensions, obtained by machining.

Electrochemical Test

The technique used to characterize the behavior of the treated test pieces with respect to corrosion is that of polarization curves. The anodic and cathodic curves were obtained on different samples for each of the studied sealing compositions.

To this end, a thermostatically controlled cell with 3 standard electrodes was used. The medium was a solution of 0.1 M NaCl in water, pH 5.67. The measurements were carried out at 25° C. The counter electrode was made of platinum and the reference electrode was made of silver/silver chloride/3M potassium chloride (E(Ag/AgCl)=+0.210 V vs standard hydrogen electrode).

Potentiodynamic anodic and cathodic polarization curves were obtained by a Gamry potentiostat/galvanostat, with a potential sweep speed of 0.5 mV/s.

The recording of the potentiodynamic curves was carried out from the potential of the open circuit (E_(ocp)), measured in the absence of external current both in the anodic and cathodic directions. Individual samples were used for each recorded potentiodynamic curve. The open circuit potential of the studied samples was established by direct measurement of the function “E_(ocp-τ)” relative to the same reference electrode after immersion in 0.1 M NaCl solution for up to 15 min. Corrosion current values i_(corr) were determined by Tafel extrapolation of the linear region of the anodic polarization curves to corrosion potential E_(corr).

For the unsealed comparative example Comp, a corrosion current i_(corr)=6.10⁻⁸ A·cm⁻² was thus obtained. The comparison of the anodic and cathodic potentiodynamic curves obtained for the test pieces before and after their anodization showed that the formed anode layer is an effective barrier for the cathodic reaction as well as for the anodic reaction of the corrosion process.

Regarding the sealed test pieces, the results obtained are shown in Table 3 below.

TABLE 3 Electrochemical parameters of anodized and sealed aluminum alloy test pieces E_(l) E_(corr) i_(corr) Composition (V vs SSC) (V vs SSC) (A · cm⁻²) C1 −0.759 −0.852 4 · 10⁻⁹ C2 −1.006 −0.986 1 · 10⁻⁸ C3 −0.877 −0.883 5 · 10⁻⁸ Comp2 −0.656 −0.639 ≥6 · 10⁻⁸ 

SSC denotes the standard silver electrode, E_(corr) denotes the corrosion potential, and i_(corr) denotes the corrosion current.

It is noted that for all of the compositions applied by the method according to the invention and tested, the corrosion current is lower than that obtained for the unsealed comparative example, as well as for the comparative example not according to the invention Comp2. Particularly good results are obtained for the compositions applied by the method according to the invention C1 and C2.

Similar results are obtained for test pieces made from the same alloy and with the same dimensions, obtained by machining.

Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX)

The morphology, structure and surface composition of the sealed test pieces were examined by scanning electron microscopy (SEM) using a JEOL JSM 6390 electron microscope (Japan) equipped with an ultra-high resolution scanning system (ASID-3D), under secondary electron image (SEI) conditions, and backscattered electron image (BEI) conditions. The electron microscope was equipped with an Oxford Instruments INCA x-sight energy-dispersive spectrometer, which enables X-ray analysis by EDX microprobe of the samples studied at a fixed point.

The results obtained by EDX analysis are shown in Table 4 below, for the full spectrum, with an amplification of ×500. For each element, the content is indicated in % by weight. It should be noted that this technique does not make it possible to detect the presence of lithium and cerium in the surface layer of the part.

TABLE 4 EDX analysis of anodized and sealed aluminum alloy test pieces Composition C1 C2 C3 C4 Comp1 Comp2 Comp Al 8.5 15.0 12.0 18.4  3.3 74.8  94.2 Cu 0.4 — — 0.4 — 0.8 3.8 Mg — — — 0.2 — 1.1 1.5 Mn — nd — nd nd nd 0.5 Zr 17.1  18.8 18.2 16.8 45.7 — — F 47.0  46.5 48.6 41.5 35.4 — — Mo 2.3 nd nd nd — 5.4 — S —  1.1  1.4 1.3 — 9.4 — K 17.9  18.7 14.8 16.3 15.7 nd — Na 6.7 nd  5.1 5.2 — nd — nd indicates an element which is present but of nondetermined quantity

FIG. 2 shows the micrographs obtained by SEM for the test pieces treated with the compositions applied by the method according to the invention C1 (in a/), C2 (in b/), C3 (in c/) and C4 (in d/).

As can be seen, the test piece treated with composition C1 (a/) is characterized by a dense surface layer with a symmetrically ordered spheroidal structure. The integral analysis from a “large” area established the presence of zirconium, fluorine and molybdenum.

The test piece treated with composition C2 (b/) is covered with a dense surface layer. It also contains uniform spheroidal agglomerates in particular containing zirconium, fluorine, manganese and molybdenum.

The test piece treated with composition C3 (c/) is also covered with a dense layer of spheroidal agglomerates of two orders of different size and of different chemical composition. The integral EDX analysis of a “large” surface notably established the presence of zirconium, fluorine and molybdenum.

For the test piece treated with composition C4 (d/), the surface morphology is also a dense surface layer containing spheroidal agglomerates, and in particular containing zirconium, fluorine, manganese and molybdenum.

Similar results are obtained for test pieces made from the same alloy and with the same dimensions, obtained by machining.

EXAMPLE 3

Rolled test pieces made from AA2024 aluminum alloy, with dimensions 150×80×3 mm, were treated by the method according to the present invention described in Example 1 above, but with the following aqueous composition according to the invention:

K₂ZrF₆ 12 g/l Na₂MoO₄, 2H₂O 10 g/l LiNO₃ 8 g/l KMnO₄ 19 g/l Ce(NO₃)₃, 6H₂O 0.2 g/l

The pH of this composition is 5.82 (measured at 19.9° C.).

The conditions for treatment with this composition are as follows: 19° C., 15 min.

At the end of the treatment, after rinsing with water and drying at 60° C., the test pieces are marked, in an X pattern, in accordance with the ISO 17872 standard, using a Van Laar tip made from tungsten carbonate. The marks are deep, so as to fully penetrate the surface layer, until they reach the basic metal alloy making up the test piece.

A photograph of a test piece thus marked is shown in FIG. 3. FIG. 4 shows, in a/, an analysis micrograph by scanning electron microscopy of a marked area.

By way of comparison, marked test pieces are also produced after anodization and sealing treatment by means of an aqueous composition containing only K₂ZrF₆ at a concentration of 12 g/l.

The test pieces thus marked are subjected to a salt spray exposure test for 816 h, at a temperature between 15 and 25° C., according to the conditions in accordance with the NF EN ISO 9227 standard.

The results obtained, for each condition tested, are shown in FIG. 4, in b/ for a test piece treated by means of the composition applied by the method according to the invention, and in c/ and d/, at different magnifications, for a test piece treated with the comparative composition containing only potassium hexafluorozirconate.

As can be seen in this figure, at the end of the salt spray exposure test, for the test piece treated with the comparative composition, containing only potassium hexafluorozirconate, pitting corrosion is observed in the marks formed on the test piece (examples of which are indicated by a box in c/ and d/ in the figure). On the contrary, no pitting corrosion was observed for the test piece treated in accordance with the present invention, and defects caused by the marking were even repaired. This clearly demonstrates the effectiveness of the composition used by the method according to the invention for the protection of parts against corrosion, and a self-healing effect of this composition: the defects induced on the surface of the part are advantageously effectively repaired.

Similar results are obtained for test pieces made from the same alloy and with the same dimensions, obtained by machining. 

1. A method for the surface treatment of a part made from aluminum or aluminum alloy or from magnesium or magnesium alloy, the method comprising: oxidation treating said part; and, applying, to the surface of said part, an aqueous composition free of chromium, the aqueous composition containing: fluorozirconate ions, molybdate ions, and at least one component selected from lithium ions and permanganate ions.
 2. The method according to claim 1, wherein said oxidation treatment of said part is an anodization treatment.
 3. The method according to claim 1 wherein said oxidation treatment of said part is a micro-arc oxidation.
 4. The method according to claim 1 wherein said application is carried out by immersing said part in a bath of said aqueous composition.
 5. The method according to claim 1 wherein said aqueous composition applied during the application has a temperature of between 10 and 60° C.
 6. The method according to claim 1 wherein said application is carried out for a time greater than or equal to 5 minutes, preferably between 5 and 20 minutes.
 7. The method according to claim 1 further comprising preliminarily pretreating said part by at least one of chemical degreasing and pickling.
 8. The method according to claim 1 wherein said aqueous composition contains lithium ions and permanganate ions.
 9. The method according to claim 1 wherein said aqueous composition contains cerium ions.
 10. The method according to claim 1 wherein said aqueous composition contains nitrate ions.
 11. The method according to claim 10, wherein said aqueous composition contains lithium nitrate.
 12. The method according to claim 1 wherein said aqueous composition has a pH of between 3 and
 7. 13. The method according to claim 1 wherein said aqueous composition contains at least one of the following concentrations: between 5 and 30 g/l of fluorozirconate salt, between 2.5 and 10 g/l of molybdate salt, between 2 and 8 g/l of lithium salt, between 4 and 19 g/l of permanganate salt, and/or between 0.1 and 0.5 g/l of cerium salt.
 14. A part made from aluminum or aluminum alloy or from magnesium or magnesium alloy obtained by a surface treatment comprising: oxidation treating said part; and, applying, to the surface of said part, an aqueous composition free of chromium, the aqueous composition containing: fluorozirconate ions, molybdate ions, and at least one component selected from lithium ions and permanganate ions. 